This invention relates to a fuel cell including a separator formed of an aluminum-based material, and a separator for a fuel cell formed of an aluminum-based material.
A fuel cell is comprised of a plurality of unit cells arranged at a pre-set distance from one another along the direction of thickness, each unit cell being made up of a positive electrode and a negative electrode constituting a pair of electrodes, and an electrolyte film sandwiched between said positive and negative electrodes. In this fuel cell, the positive electrode faces a positive electrode chamber fed with an active material for the positive electrode, while the negative electrode faces a negative electrode chamber fed with an active material for the negative electrode. In this fuel cell, the negative electrode chamber, fed with the active material for the negative electrode, is partitioned by a separator from the positive electrode chamber, and fed with the active material for the positive electrode.
In the fuel cell, the separator undergoes corrosion and deterioration dep ending on the environment in which the cell is used. Thus, with the fuel cell, power generating characteristics after prolonged use for power generation tends to be lowered in comparison with the initial power generating characteristics. Thus, the separator is customarily prepared form a carbon material less susceptible to deterioration due to corrosion. There has hitherto been known a fuel cell formed of stainless steel or titanium which forms a strong inactivated film retained to exhibit resistance against corrosion.
However, in the course of the investigations toward the present invention the following problems have been encountered. Namely, the separator formed of a carbon material is highly expensive because of the high cost of the material itself. In addition, since the material is brittle, the separator is increased in thickness, thus restricting reduction in size of the fuel cell.
The separator formed of stainless steel, while being meritorious for size reduction because of its superior strength, is not meritorious for reduction in weight because of the high specific gravity of the material. On the other hand, the separator formed of titanium, which is meritorious for size reduction because of its superior strength, is not meritorious for cost reduction because of its high material cost, nor sufficient for weight reduction because of its specific gravity.
Recently, the present Assignee has conducted research and development of a separator formed of an aluminum based material of a low specific gravity for reducing the weight and the cost of the separator.
In the separator constituting a fuel cell, since an electrically conductive path is produced along its direction of thickness, it is not desirable that an oxide film be formed on a surface layer of an aluminum-based material constituting the separator. It is therefore not desirable to process the separator formed of the aluminum-based material with anodic oxidation to generate an anodic oxide film. Thus, the fuel cell with a built-in separator formed of the aluminum-based material is not sufficient in durability against corrosion.
In view of the above-depicted status of the art, it is an object of the present invention to provide a fuel cell and a separator for the fuel cell meritorious in reducing the weight and improving resistance against corrosion.
The present inventors have conducted eager searches into developing a separator for a fuel cell formed of an aluminum-based material, and found that, if the separator is a of a layered structure comprised of an aluminum-based substrate, at least one intermediate plating layer layered on the aluminum-based substrate and a noble metal layer layered on the intermediate plating layer, with the intermediate plating layer being mainly composed of at least one of zinc, copper and tin, it becomes meritoriously possible to reduce the weight of the separator, to improve resistance against corrosion, to procure adherent power of the noble metal plating layer and resistance against corrosion of the noble metal plating layer and to reduce the resistance to electrical resistance in the thickness direction. This information, confirmed by tests, led to development of the separator and the fuel cell according to the present invention.
A fuel cell according to a first aspect of the present invention includes a plurality of unit cells arranged at a pre-set distance from one another along the direction of thickness, each unit cell being made up of a positive electrode and a negative electrode constituting a pair of electrodes, and an electrolyte film sandwiched between the positive and negative electrodes, and a plurality of separators, each arranged between neighboring ones of the unit cells for separating a negative electrode chamber facing the negative electrode and a positive electrode chamber facing the positive electrode. The negative electrode chamber and the positive electrode chamber are fed with an active material for the negative electrode and with an active material for the positive electrode, respectively. The separator includes an aluminum-based substrate, at least one intermediate plating layer layered on the aluminum-based substrate and a noble metal layer layered on the intermediate plating layer. The intermediate plating layer is mainly composed of at least one of zinc, copper and tin.
With the fuel cell according to the present invention, since the separator is mainly formed of an aluminum-based substrate, it can be reduced in weight. Moreover, since various plating layers are layered in the above-described order on the aluminum-based substrate constituting the separator, it is possible to suppress corrosion and deterioration of the separator.
According to a second aspect of the present invention, there is provided a separator for a fuel cell, i.e., a separator for partitioning a negative electrode chamber fed with an active material for the negative electrode and a positive electrode chamber fed with an active material for the negative electrode from each other. The separator includes an aluminum-based substrate, at least one intermediate plating layer layered on the aluminum-based substrate and a noble metal layer layered on the intermediate plating layer. The intermediate plating layer is mainly composed of at least one of zinc, copper and tin.
Since the separator for the fuel cell according to the present invention is mainly comprised of an aluminum-based substrate, it can be reduced in weight. Moreover, since various plating layers are layered in the above-described order on the aluminum-based substrate constituting the separator, it is possible to suppress corrosion and deterioration of the separator.
According to the present invention, a well-known aluminum-based material may be used as a material for an aluminum-based substrate constituting separator. For example, pure Al, Alxe2x80x94Mg, Alxe2x80x94Si, Alxe2x80x94Mgxe2x80x94Si, Alxe2x80x94Mn or Alxe2x80x94Zn based materials may be used, i.e., Al or Al alloys.
According to the present invention, the intermediate plating layer may be constituted by at least one of a zinc plating layer, a copper plating layer and a tin plating layer. The zinc plating layer is preferably a zinc substitution plating layer for securing adherent power to the aluminum-based substrate. The intermediate layer may, for example, be made up of a zinc substitution plating layer, layered on the- aluminum-based substrate, and a copper plating layer, layered on the zinc substitution plating layer. Typical of the noble metal layers is a silver plating layer in view of electrical conductivity and material cost. The plating herein means coating a metallic material and the plating method includes plating processing such as chemical plating and electrical plating.
If two or more layers are used as the intermediate plating layer, the layers are preferably arranged in the order of decreasing ionization tendency from the aluminum-based substrate side.